Lightning protection for antenna systems

Abstract: A wave transmission line (10, 16) offering protection for electronic devices connected to the line from the effects of a high voltage impulse of relatively low frequency which includes a wave propagating line connected to at least one of the devices. The line is characterized in that it includes at least one segment which is a waveguide (16) dimensioned to allow free passage of waves of relatively high frequencies during normal operation while substantially preventing the propagation of the relatively low frequency waves of the impulse.

Description: The present invention relates to means for protecting electrical components from the damaging effects of lightning and, more particularly, to means for protecting a receiver or transmitter front end, which may be an amplifier, a pre-amplifier, mixer, detector, filter, limiter, T/R switch and the like, connected to an antenna, from being damaged when lightning strikes the antenna. Typical antenna systems, such as those used in the transmission and reception of microwave signals, include an antenna dish mounted in a prominent position, usually on a mast and/or on top of a building, where it can achieve line-of-sight contact with other antennas. The antenna is connected by a transmission line, typically a coaxial line or cable, to a front end located in the vicinity of the antenna, and from there to the rest of the components of the system. The placement of the antenna in a prominent location, where it is often among the tallest structures in the vicinity, makes it highly vulnerable to attack to a lightning strike. When lightning strikes the antenna dish, the coaxial line directs the high voltage impulse to the front end and, typically, destroys it, rendering the system inoperable and necessitating the replacement of the damage component. Various attempts have been made to protect the front end from lightning strikes. One such technique is described in "Protecting VHF antennas from EMP", S. Sebastian, I.E.E.E., 1991, CH-3044-5, and involves use of an appropriately rated spark gap between the inner and outer conductors. During normal operation, the two conductors are isolated from each other by the dielectric. However, when a sufficiently large voltage is encountered, a short circuit is created in the spark gap which prevents the impulse from reaching the front end. A disadvantage of the spark gap technique is that it is limited by capacitance considerations to use at relatively low frequencies. GB-A-736,365 teaches a high frequency coupling arrangement comprising a non-metallic dielectric waveconductor (i.e. a non-metallic waveguide) and means for feeding and taking high frequency energy from the non-metallic dielectric waveconductor, which prevents passage of low frequency coupling arrangement. JP-A-01 278 101 teaches a wave propagating line including a metallic waveguide line and, like GB-A-736,365, an insulating waveguide implemented therein. JP-A-01 278 101 further teaches grounding of the wave propagating line on the antenna's side to create a low resistivity path to ground. Despite these disclosures, there remains a widely recognised need for, and it would be highly advantageous to have, a simple means for protecting components connected to an antenna from the damaging effects of lightning which means would be operable over a wide range of frequencies. According to the present invention there is provided an electronic protection system comprising at least one electronic device connected to a wave propagating line at least one segment of which is a coaxial line and at least one other segment of which is a wave conductor to protect the device or devices from the effects of a high voltage impulse of relatively low frequency, characterised in that the at least one other segment is a waveguide having an electrically conductive guide wall extending from its input to its output and dimensioned to allow free passage of waves of relatively high frequencies during normal operation while substantially preventing the propagation of the relatively low frequency waves of the impulse. According to the preferred embodiments of the invention described below, the waveguide is either rectangular or circular in cross-section. The present invention successfully addresses the shortcomings of the presently known configurations as it is able to provide a simple solution to the problem of damage from lightning to electrical components connected to antennas and similar devices. While the description herein is concerned mainly with the protection of a front end connected to a microwave antenna against the detrimental effects of a lightning strike, it will be readily appreciated that the present invention can be applied in a variety of systems wherein it is desired to isolate an electronic component from the detrimental effects of a high voltage impulse of relatively low frequency. The present invention discloses a novel wave propagating line which includes at least one segment made up of a waveguide of electrically conductive material. The waveguide is characterised in that it substantially prevents the transmission of waves below a certain frequency, depending on the dimensions of the waveguide. The presence of the waveguide in a transmission line, such as a conventional coaxial line, serves to isolate electronic equipment from lightning. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- FIG. 1 is a schematic representation of one embodiment according to the present invention; FIG. 2 is a perspective partial cutaway view of a typical coaxial line; FIG. 3 is a perspective view of a waveguide having a rectangular cross-section; FIG. 4 is a perspective view of a waveguide having a circular cross-section. The present invention is a means for protecting electronic equipment from the detrimental effects of certain high voltage impulses. Specifically, the present invention can be used to isolate a front end connected to a microwave antenna from the effects of lightning strikes. Transmissions lines are available in many forms. The most important and widely used of all the many possible transmission lines is the coaxial line, or coaxial cable, as it is sometimes called when used with flexible conductors. In a conventional microwave antenna system, the antenna is connected to a nearby amplifier, mixer, detector, and the like, through a suitable coaxial cable. A disadvantage of such a system is that a lightning strike of the antenna can destroy the front end. The principles and operation of a system according to the present invention may be better understood with reference to the drawings and the accompanying description. Referring now to the drawings, Figure 1 illustrates a typical system, according to the present invention. A transmission line 10 , such as a coaxial line, is used to connect, for example, a microwave antenna 12 , or portion of a compound antenna made up of a series of elements, and a front end which is made up, for example, of an amplifier 14 . The gist of the present invention is that transmission line 10 does not directly connect microwave antenna 12 and the front end, such as, for example, amplifier 14 . Rather, a waveguide 16 of suitable dimensions, as described below, is connected in series with transmission line 10 . The waveguide is conventional in that it is a metallic waveguide, i.e. it has an electrically conductive guide wall or walls extending from its input to its output. The connection may be as in Figure 1 with waveguide 16 connected at both ends to segments of coaxial lines 10 . Alternately, waveguide 16 may be connected at one end to either antenna 12 or amplifier 14 and to transmission line 10 at the other. In other embodiments, more than one segments of waveguide 16 may be used. The combination of one or more segments of waveguide 16 connected in series with one or more segments of transmission line 10 together forms a wave transmission line. A typical coaxial line 10 is shown in Figure 2. Coaxial line 10 includes a cylindrical inner conductor 18 of radius 'a' which is wrapped by annular dielectric 20 of outer radius 'b' insulating inner conductor 18 from annular outer conductor 22 which is, in turn, covered by an insulating jacket 24 . Two typical examples of waveguides 16 are shown in Figure 3 and 4 which depict a waveguide of rectangular cross section having a width 'a' and height 'b' and a waveguide of circular cross section having a radius 'a', respectively. The waveguide may be of any suitable design, including, but not limited to, single-ridged and double-ridged waveguides. Both transmission lines, such as coaxial lines 10 , and waveguides 16 propagate waves by modes. However, in contrast with transmission lines, such as coaxial lines 10 , which operate at any frequency up to a maximum, or cutoff value, waveguides 16 have mininum, as well as maximum, cutoff frequencies. The present invention takes advantage of the fact that waveguides have minimum cutoff frequencies. For example, it is known that in the common rectangular waveguide (Figure 3) the lower cutoff frequency, f c , above which normal operation must occur if operation is to be at the lowest, or dominant, mode, is: f c = 1/[(2a)(µε) -0.5 ] where µ and ε are the permeability and the permittivity (dielectric constant), respectively, of the dielectric filling the guide. When air is used as the dielectric, Equation (1) reduces to: f c = c/(2a) where c is the speed of light in vacuum. As another example, in a circular waveguide (Figure 4) the lower cutoff frequency, f c , is f c = 0.2931/[a(µε) -0.5 ] When air is used as the dielectric, Equation (3) reduces to f c = 0.293c/a Thus, it is possible to select a waveguide of a particular width or radius which will not allow waves of relatively low frequencies, such as the high voltage impulse of up to 100 MHz frequencies which are typical for lightning, to pass, yet allow the higher frequency waves encountered in normal operation to pass through unhindered. The waveguide may be of any suitable height and length, which may be selected so as to reduce losses at the transitions between the waveguide and the coaxial line. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims: An electronic protection system comprising at least one electronic device (14) connected to a wave propagating line (10) at least one segment of which is a coaxial line and at least one other segment of which is a wave conductor (16) to protect the device or devices from the effects of a high voltage impulse of relatively low frequency characterised in that the at least one other segment is a waveguide (16) having an electrically conductive guide wall extending from its input to its output and dimensioned to allow free passage of waves of relatively high frequencies during normal operation while substantially preventing the propagation of the relatively low frequency waves of the impulse. A system according to claim 1 wherein said waveguide is rectangular in cross-section (Fig. 3). A system according to claim 1 wherein said waveguide is circular in cross-section (Fig. 4). A system according to claims 1, 2 or 3 wherein the line includes a single segment (16) of said waveguide connected at both ends to wave propagating line (10). A system according to any one of the preceding claims wherein the line is connected to an antenna (12) to protect against a high voltage impulse of relatively low frequency generated by lightning. A system according to claim 5 wherein said antenna (12) is a microwave antenna. A system according to any one of the preceding claims wherein one of said electronic devices (14) is an amplifier. A system according to any one of the preceding claims wherein the waveguide (16) is dimensioned substantially to prevent the passage of waves up to 100 MHz.

Inventors: HARTAL OREN, MILSTEIN ELIEZER, HARTAL, OREN, MILSTEIN, ELIEZER
Applicants: ISRAEL STATE, STATE OF ISRAEL - MINISTRY OF DEFENCE
Application year: 1994
Main classification: H01Q1
IPC codes: H01Q1